“Four-twenty” is a well-known code phrase in the pot-smoking culture for the consumption of cannabis and refers to both the time of day and the date. On April 20 (or 4/20) pot smokers have held public rallies, including at the University of Vermont, where they openly imbibe and call for the legalization of marijuana.
Sorrell is well aware of the connection and said he laughed when he first saw the plates. He said he smiles every time he gets into his Jeep and has gotten a lot of thumbs-up from people in other vehicles.

tl;dr: Basically, it’s all former Gov. Howard Dean’s fault.

Some people thought the former governor had picked the number as a joke, Sorrell said, but Dean didn’t see the connection between the number 420 and pot consumption until Sorrell reminded him of the April 20 protests.